Africa's emerging virus threat

Previously uninhabited areas could harbour viruses

Many of the most frightening illnesses to emerge in the modern era appear to have originated from Africa.

But scientists are still unsure why these previously unknown illnesses have emerged from different parts of the continent.

The latest outbreak of the Ebola virus in Uganda may be the first to arise in a densely populated area, which is alarming doctors trying to keep it under control.

Ebola is not only infectious, but has no treatment and is fatal in a large proportion of cases, depending on the strain involved.

So far, almost 40 people have died in the latest Ugandan outbreak.

But Ebola, while the most notorious, is not the first so-called haemorrhagic fever to have its roots in Africa.

Lassa Fever, Rift Valley Fever and the Marburg virus can all be traced back there in one form or another.

Not just Africa

However, it is not just Africa which has seen the emergence of previously unheard-of viruses over the past few decades.

New viruses, including those which cause haemorrhagic fevers, have come to light in Bolivia, Argentina, Columbia and Brazil.

Dr Diana Lockwood, a consultant physician at the Hospital for Tropical Diseases in London, said that an extra factor in Ebola was the fact that it could be spread from person to person.

She said: "We have syringes being reused, poor hospital hygeine - it's poverty really."

Aids virus

And despite ongoing controversy about its origins - and the reason for its jump into humans - the biggest killer of all, the Aids virus, is also thought to have an African source.

More and more rainforest is being logged

In history, Africa certainly has no monopoly on the emergence of "plagues" - the bacillus responsible for the Black Death, which killed many millions around the world in the medieval era, is thought to have come first from the Gobi Desert area in central Asia.

In just a few years, the population of Europe fell by a third - some 25m people.

It is tempting to think that these are brand new viruses - but the evidence suggests that they are much older than that.

A common explanation embraced by many scientists, is that the spread of human civilisation into previously uninhabited areas brings them into contact with natural "reservoirs" of viruses which have been there for millenniae.

It is likely they live in certain species of animal while causing them no harm.

However, as new villages or industry springs up, perhaps humans are far more likely to come into contact with these viruses.

The virus would be kept away from human contact by the simple fact that most animals - like monkeys - infected from the reservoir would die quickly.

Hunting the reservoir

Ebola was first identified in equatorial Sudan following significant outbreaks in northern Zaire (now Democratic Republic of Congo) and southern Sudan in the mid 1970s.

There have been other cases since in Ivory Coast and Gabon.

Extensive ecological studies are underway in three African countries, including DR Congo, to try to find the "reservoir" - this work essentially involves checking for signs of the virus in thousands of different species of animal, in order to identify the carrier.

Certain bats infected in the laboratory with Ebola has shown they do not die - leading to speculation that they may play a role.

However, until the ecological studies are completed, scientists will still be very much in the dark about the origins of this mysterious illness.

The circumstances surrounding the emergence of HIV are far more controversial.

Scientists believe that human Aids is a disease which has been passed from animals to humans - in this case monkeys.

The equivalent virus in monkeys is called SIV, or simian immunodeficiency virus.

Africa is the only place in which wild monkeys carry SIV - which points the finger at the continent as the source of the infection.

Just how the virus jumped the species barrier, from monkeys to humans, is a matter of debate - it could be through human consumption of monkey tissue, or, as some have suggested, the injection of human test subjects with monkey blood during malaria experiments in the US and Europe during the 1950s.